This invention relates to the field of Semiconductor manufacture. More particularly, it relates to the monitoring of plasma charging during plasma processing.
Plasma processing has become an integral part of the fabrication of integrated circuits, because it offers advantages in terms of directionality, low temperature and processing convenience. For sub-micron VLSI manufacturing, plasma etching is essential for pattern definition.
Effects of Charging During Plasma Etching
However, substrate charging during plasma etching processes is a concern as it can result in both profile distortion and thin oxide damage. Etching-induced profile distortions, including tilting, bowing, undercutting and notching, may be caused by wafer surface charging. These distortions have been shown to be a serious problem particularly in sub-micron features.
As gate oxide thicknesses have decreased in an effort to improve MOS device performance, oxide damage has been experienced after various plasma processing steps, with etching and ashing being of most concern. This damage can degrade essentially all the electrical properties of a gate oxide, which include the fixed oxide charge density, the interface state density, the flat band voltage, the leakage current and the various breakdown-related parameters. In short, all the MOS transistor parameters which depend on the oxide parameters can be degraded as a result of the charging that results from plasma processing. It is, therefore, critical to monitor wafer charging during plasma processing.
The primary damage mechanism attributed to charging during plasma processing appears to be charging of the gate to a magnitude approaching the oxide breakdown voltage. When this condition occurs, conduction of sufficient Fowler-Nordheim (F-N) tunneling current is induced through the gate oxide to create enough charge traps to affect the electrical characteristics of the oxide.
In general, plasma-charging-induced device damage is characterized by the so-called xe2x80x9cantenna dependencexe2x80x9d relationship. In the context of charging damage, an xe2x80x9cantennaxe2x80x9d refers to any conductor connected to a floating gate. An antenna increases thin oxide current and thus damage by increasing the collection of unbalanced particle current from the plasma. As the charge collecting antenna area increases, so does the tunneling current due to the accumulation of additional charges.
Background on Plasma Charging Monitoring
Plasma charging monitoring has taken on several forms. These, however, follow one of two broad methodologies: indirect characterization of plasma charge damage and direct monitoring.
With respect to the indirect characterization methodology, two different approaches have been used. In the first approach, the plasma damage is evaluated without extra manufacturing processes and damage is characterized by comparison to a wet-etched control device. The second approach employs already-manufactured devices which are exposed to processing plasma. Damage is evaluated by comparing the subject device properties before and after exposure to plasma. Both of these approaches are limited in effectiveness since they both involve characterization after plasma processing has taken place. In the aforementioned methods for indirect characterization of plasma charging, the surface potential is derived from the threshold measurement of EEPROM devices, circuit modeling of observed damage and from ion trajectory simulations for profile distortions. However, none of these derivations provide any way to distinguish whether the damage caused is due to transient behavior resulting from plasma on-off, or steady state charging. Measurement of plasma potential made possible by techniques such as the Langmuir probe technique are similarly deficient in the information derived since from these techniques, it is not clear that plasma potential can absolutely relate to charging voltage due to the complicated plasma sheath and the wafer surface profile contour. In this regard, electron shading in particular can cause a local charging effect.
It is therefore necessary and desirable to measure plasma charging voltage directly on the wafer surface during processing to permit etcher hardware modification and process development problem solving.
In this regard, direct methods of monitoring plasma charging in real time on a wafer surface have been developed. Generally these methods employ a sensor comprising one or more MOS capacitors manufactured on top of a wafer surface. The MOS structure is compatible with real wafer processing and the geometry of the antenna structure in an actual MOS transistor can be replicated. In this methodology, the plasma induced charging potential is measured outside the chamber during the plasma etching or deposition process.
An arrangement typical of this method is shown in FIG. 1. The sensor of FIG. 1 consists of a 4 inch Si wafer to which DC connections were made to both a large exposed Al pad on top of a 1 xcexcm oxide and to the substrate. Contacts are made to the Al pad and substrate via on-wafer Al leads connected to flexible Cu wires at the wafer edge. All the Al leads and Cu wires are encapsulated by epoxy resin to prevent their exposure to the plasma and the possible perturbation that could result. FIG. 1 is discussed in further detail below.
In this methodology, the charging voltage, Vc, is defined as the DC voltage developed between the pad and the substrate. By providing real-time monitoring of the variation of charging voltage, this methodology has shown that plasma charging voltage is a function of power, pressure, gas species, magnetic field and transient time. This type of sensor has also proven to be a good diagnostic tool for equipment or process development.
The configuration of an in-situ plasma charging sensor as described above, however, has several drawbacks. For example, in these sensors, all electrical connections to the sensor and the leads connecting the sensor to monitoring equipment outside the plasma chamber are epoxy coated to prevent their exposure to the plasma. There is always a risk that the epoxy coating could be breached, thereby exposing the leads or connections to the plasma, and causing unwanted perturbation of the plasma. Perturbation of the plasma could result in inaccurate measurements of plasma charging, thereby defeating the sensor""s purpose. Moreover, the layout of the sensor which necessitates the running of epoxy coated leads through the wall of the plasma chamber is inherently fragile and prone to compromise particularly where the sensor is applied to field testing of plasma etching processes and process development.
These and other drawbacks of the prior art are addressed by the present invention. According to a first embodiment of the present invention, an apparatus for the monitoring of plasma charging comprises an electrostatic chuck assembly in an etch chamber. The electrostatic chuck assembly is provided with at least two holes through which spring loaded lift pins project. The spring loaded lift pins are modified to contain electrical contacts at their ends, which project through the holes provided in the electrostatic chuck assembly.
A detection wafer is removably mounted on the electrostatic chuck assembly by electrostatic force. The detection wafer comprises a semiconductor substrate layer having a hole provided in it that aligns with one of the holes provided in the electrostatic chuck assembly. The detection wafer further comprises a dielectric layer disposed over the semiconductor substrate layer and one or more metal electrodes extending over a portion of an upper surface of the dielectric layer and extending through the dielectric layer such that the metal electrode does not contact the semiconductor layer but is accessible from a lower side of the semiconductor substrate layer via the hole in the semiconductor substrate layer. The wafer is fabricated using a series of masks with conventional processing.
When the detection wafer is placed on the electrostatic chuck and retained by electrostatic force during processing, the electrical contact in the end of one of the lift pins projecting through the holes in the electrostatic chuck is mechanically biased against the lower side of the semiconductor layer, thereby establishing electrical contact with the semiconductor layer through one hole in the electrostatic chuck assembly. The second lift pin is mechanically biased against the portion of the electrode extending through the first dielectric layer by projecting through the second hole in the electrostatic chuck assembly and the hole in the semiconductor layer accessible through the lower side of the semiconductor substrate layer respectively, thereby establishing another electrical contact. A voltage measuring apparatus is connected to the lift pins in electrical contact with the electrode and semiconductor substrate respectively and measures the differential voltage between the electrode and semiconductor substrate when the detection wafer is exposed to a plasma.
Accordingly, one advantage of the present invention is that it provides a method, system and apparatus for plasma charging voltage measurement that does not require contacts and leads to be exposed to, or routed within the plasma processing environment during the etching process.
Another advantage of the invention is that it provides a method, system and apparatus for plasma charging monitoring that does not require the use of a conducting wire between the wafer and dielectric etch chamber wall during measurement, thereby eliminating the potential of perturbation of the plasma by a conducting wire. This arrangement eliminates the potential for arcing between the wafer and the measurement wire or chamber wall that exists in typical setups.
Another advantage of the invention is that it eliminates wire bonding between a detection wafer and contacting wire.
Another advantage of the invention is that all measurement contacts are made through a modified lift pin assembly requiring no special modifications to an electrostatic chuck assembly.
Another advantage of the invention is that the detection wafer used to detect plasma charging can be reused. For example in dielectric etch, the wafer can be reused after simple resist stripping and lithography process steps thereby reducing the overall cost of the use of monitoring wafers by recycling them.
Another advantage of the invention is that the DC bias on the detection wafer, plasma density uniformity and plasma properties including ion saturation current, plasma potential and electron temperature can also be measured or calculated from the same setup.
Another advantage of the invention is that the dielectric materials used on the detection wafer can be non-Si and non-SiO2 materials such as AlN, Al2O3, SiC and Ta2O5, thereby extending the wafer measurement lifetime since these materials have a longer erosion rate than Si and SiO2.
Another advantage of the invention is that the environment in which monitoring of plasma charging occurs, is identical to that which is present during semiconductor manufacture, thereby providing an accurate representation of manufacturing conditions.
Yet another advantage of the invention is that it provides a method, system and apparatus for plasma charging voltage measurement and monitoring that permit easy installation and use in field applications without the risk of compromising the integrity and accuracy of the measurement apparatus for the diagnosis of problems in the field.
These and other embodiments and advantages of the present invention will become immediately apparent to those of ordinary skill in the art upon review of the remainder of the specification and the claims to follow.